1. Field of the Invention
The invention is generally related to new, dense, homogenous, crack-free ceramic oxide coatings and foams, and to new methods for forming these coatings and foams.
2. Description of the Prior Art
Ceramics in general are well known for their thermal stability, and often exhibit excellent resistance to wear and corrosion as well. Therefore, a great number of ceramic materials are candidates for applications in high-temperature, extremely corrosive environments encountered in a wide range of industries. Particularly challenging to the reliability of structural ceramic components are those applications requiring good resistance to attack by alkalies. These applications include: refractories subjected to the action of alkali vapors or slag in gas furnaces, blast furnaces and stove construction, cement kiln linings, combustion chamber boilers, and town gas installations; advanced high temperature coal conversion and combustion; heat exchangers; and other energy systems. It has been found that many ceramics can be attacked rapidly by alkali; therefore, the protection of ceramics from alkali corrosion is an important problem.
Currently, many materials in high temperature service are performing at their capability limits. As material requirements become increasingly sophisticated, it is becoming more and more difficult to combine the required structural properties and stability in a single material. The application of a high temperature thermal barrier and corrosion resistant material to substrates which possess the required mechanical properties for a specific application can produce cost-effective composite systems which optimize both corrosion resistance and strength.
For example, since non-oxide ceramics such as silicon carbide and silicon nitride inherently possess outstanding fracture strength to high temperature and have excellent thermal shock resistance and corrosion resistance in air at high temperatures, they can be fabricated into required shapes and sizes for structure applications at high temperatures. However, both silicon carbide and silicon nitride corrode severely in industrial furnace atmospheres containing alkali compounds. Therefore, applying ceramic oxide coatings which are more resistant to alkali compounds on the silicon carbide or silicon nitride substrates should produce a composite structure with both the superior thermal properties of silicon carbide and silicon nitride and the superior alkali resistance of the ceramic oxide coating.
In the past several years, several new materials having good alkali resistance have been developed including (Ca.sub.x, Mg.sub.1-x)Zr.sub.4 (PO.sub.4).sub.6 (CMZP), ZrP.sub.2 O.sub.7, Zr.sub.2 P.sub.2 O.sub.9, and the like.
Many coating technologies can be used to prepare a protective layer of ceramic materials. However, when applying alkali corrosion resistant coatings having complex composition onto ceramic elements having complex shapes (e.g., ceramic filters), using chemical vapor deposition (CVD), chemical vapor infiltration (CVI), and plasma methods may not be satisfactory. In addition, coating adhesion to the underlying ceramic material is problematic.
In recent years, it has been found that ceramic foams having an open cell structure have a wide variety of potential applications. For example, ceramic foams may be used for thermal insulation, as structural members requiring light weight and specific stiffness, as catalyst supports, as high temperature filters for gas and liquid metals, as dispenser cathodes, as diffuser plates, as part of mass transfer equipment, in conjunction with heater elements, or in many other applications. Ceramic foams having an open cell structure have been made in the past using slurry, CVI, and CVD methods. However, for the slurry method, it has been difficult to make ceramic foams having fine open cell structure, such as foams having pore diameters of less than 250 .mu.m and a porosity higher than 87%. In addition, for CVI and CVD methods, it is difficult to produce ceramic foams having the necessary complex composition, and it is very expensive.
Brown et al. U.S. Pat. No. 5,102,836 discloses a prior invention by the applicants which is related to sol-gel and slurry processes for forming CMZP materials including CMZP foams. The CMZP materials were formed from inorganic precursor compounds, such as Mg(NO.sub.3).sub.2, Ca(NO.sub.3).sub.2, ZrO(NO.sub.3).sub.2, and NH.sub.4 H.sub.2 PO.sub.4, where the pH of a solution containing the compounds was adjusted to a range between pH 7-9, and water in the solution was evaporated to create a gel, then calcined to form CMZP powder. The CMZP powder was made into samples and sintered to form CMZP products. Slurry methods were used to produce CMZP foams. Using these slurry methods, the foams produced did not achieve a porosity greater than 87% and did not have pore diameters less than 250 .mu.m.